Once you have Speech to level 70, you just need to purchase the Investor perk in the Speech skill tree.

Once you have the Investor perk, you will see a new dialogue option when speaking to shop owners to help them out and give them 500 gold. This will permanently increase their gold limit, and you will gain some favor with them as well.

Investing into shops also counts as helping people out when trying to become a Thane of a region.